1. Field
Example embodiments relate to a method of manufacturing amorphous NiO thin films and nonvolatile memory devices including amorphous thin films that use a resistance material. Other example embodiments relate to a method of manufacturing amorphous NiO thin films having improved switching and resistance characteristics by reducing a leakage current and non-volatile memory devices using an amorphous NiO thin film.
2. Description of the Related Art
FRAMs, MRAMs, and PRAMs are nonvolatile memory devices that may use a conventional resistance material. While DRAMs or flash memories store binary information using an electric charge, FRAMs may use the polarization of a ferroelectric material. MRAMs may use the resistance change of a magnetic tunnel junction (MTJ) thin film according to the magnetization state of the ferromagnetic material and PRAMs may use the resistance change according to a phase change. FRAMs, MRAMs, and PRAMs are devices that may replace conventional volatile and nonvolatile memory, because FRAMs, MRAMs, and PRAMs may have both higher integration characteristics common to DRAMs and the nonvolatile characteristics of flash memories.
A phase-change RAM (PRAM) may be an example of a nonvolatile memory device and may store binary information using the phase change characteristics of a phase change material (e.g., GeSbTe (GST)). The phase of GST may change into a crystalline state and/or an amorphous state by localized electrical pulse heating. The PRAM may include a memory cell having a phase change layer, a resistor and a switch transistor. The phase change layer may be formed of a GST based material (e.g., chalcogenide). The purpose of the resistor may be to heat the phase change layer. The phase change layer may change the phase into a crystalline state and/or an amorphous state according to the degree of heating. The crystalline state and the amorphous state may have different resistance. The state may determine the voltage and current across the phase change layer, to allow storing and reading of binary information using the resistance difference.
FIG. 1 is a diagram illustrating a nonvolatile memory device that uses a crystalline NiO thin film manufactured using a conventional sputtering method. FIG. 2 is a graph showing the switching behavior of the crystalline NiO thin film of FIG. 1. FIG. 3 is a graph showing the resistance characteristics of a crystalline NiO thin film formed to a thickness of 200 Å using a conventional sputtering method.
Referring to FIG. 1, a nonvolatile memory device that uses a NiO thin film may include a transistor switch 20 and a data storage unit 28 connected to the transistor switch 20. The transistor switch 20 may include a source 12s, a drain 12d, a channel 12c and a gate electrode 14. The data storage unit 28 may include an upper electrode 26, a lower electrode 24 and a NiO thin film 25 interposed therebetween. An insulating layer 30 may be formed on the transistor switch 20 and the data storage unit 28 may be located in the insulating layer 30. The data storage unit 28 may be connected to the transistor switch 20 via a conductive contact plug 22.
The conventional NiO thin film 25 resistance material may be formed by sputtering and the NiO thin film 25 formed in this way may be polycrystalline. Reducing the reset current may be limited, because the NiO thin film 25 may have a larger leakage current and poorer resistance characteristics. To have a switching characteristic in a non-volatile memory device when the crystalline NiO thin film 25 is used as a resistance in the nonvolatile memory device, the crystalline NiO thin film 25 may be formed relatively thick, for example, from about 500 Å to about 1000 Å. Higher integration of the memory device may become more difficult.